1. Technical Field
This invention relates generally to sliding-type bearings, and more particularly to those having a powder metal bronze bearing material applied to a steel backing, such as used in engine bearings.
2. Related Art
It is common in engine bearing applications to bond a powder metal bronze alloy to a steel backing to journal a crankshaft or the like. The copper tin matrix provides a strong bearing surface that can withstand the loads subjected on the bearing in use. Such bearings must also exhibit suitable wear and seizure resistance properties, and for this purpose it is common to add a certain amount of lead to the bronze matrix which serves as a lubricant to the bearing surface. It is also common to add a flash coating of tin to the running surface to further enhance the wear and seizure characteristics of the bearing.
Due to environmental considerations, various substitutes for lead have been explored, but to date none have demonstrated the ability to truly substitute for lead without unduly sacrificing the strength, wear and seizure properties of engine bearings. It is an object of the present invention to provide a suitable lead-free bronze engine bearing that exhibits comparable or better properties than those containing lead.
Applicants"" have found that bismuth, when alloyed with powder metal bronze in a controlled amount along with a controlled amount of phosphorus provides a resultant steel-backed engine bearing whose physical properties are equal to or better than that of bronze lead bearings while also exhibiting wear and seizure resistant properties equal to or exceeding those of steel-backed powder metal bronze-lead engine bearings.
An engine bearing constructed according to the present invention comprises an essentially lead-free powder metal bearing material bonded to a steel backing. The bearing material consists essentially of 8 to 12% by weight of tin, 1 to less than 5% by weight of bismuth, and 0.03 to 0.8% by weight of phosphorus, with the balance being made up essentially of copper.
Engine bearings constructed according to the invention exhibit physical properties of tensile strengthxe2x89xa7400 MPa, yield strengthxe2x89xa7290 MPa, elongationxe2x89xa710% and hardnessxe2x89xa7130 Hv 0.5/15. By way of comparison, a traditional copper-tin-lead bearing having 10 wt. % tin, and 10 wt. % lead exhibits, on average, a considerably lower yield strength of 223 MPa, a comparable tensile strength of 301 MPa, a reduced elongation of about 8%, and a reduced hardness of about 96 HV 0.5/15. By way of further comparison, an identical engine wear test was conducted on bearings prepared according to the present invention against traditional copper-tin-lead bearings of the type described above. The traditional copper-tin-lead engine bearings exhibited a loss of about 12 microns due to wear, whereas bearings prepared according to the invention exhibited an average of about 10-11 microns, demonstrating that the wear and seizure resistance of bearings according to the invention are at least as good as if not better than that of the traditional copper-tin-lead engine bearings.
It has been surprisingly found that bearings prepared according to the present invention exhibit the beneficial property, when subjected to frictional sliding loading in use, of causing a certain amount of tin, which starts out being uniformly dissolved in a copper to serve as a copper-tin matrix, to migrate to the bearing surface, with the result being that a considerably high concentration of tin is developed at the bearing surface which was not present at the time the bearing was made and installed in a use application. This migration of tin and formation of a highly tin-rich layer at the bearing surface greatly increases the lubricity of the bearing and thus contributes to enhanced wear and seizure resistant characteristics of the bearing once the bearing is put into use. Such tin migration has not been observed in traditional copper-tin-lead bearings, nor with other proposed lead substitutes, such as nickel. While not entirely understood, it is believed that, when subjected to frictional sliding loading, the bismuth reacts with the tin in the matrix and effectively mobilizes the tin, drawing it to the bearing surface. Following testing, a visual inspection of the engine bearings prepared according to the invention showed the bearing surface to have a lustrous, tin-colored bearing surface, and a chemical analysis conducted on the bearing showed a considerably higher concentration of tin at the surface than in the rest of the copper-tin matrix below the surface, which remained uniform in its tin concentration.
This surprising property of tin migration has the benefit of eliminating or minimizing the need to apply a tin flash coating to the bearing surface prior to putting the bearing into service. The elimination of the flash coating step saves time and equipment and simplifies as well as lowers the cost of making engine bearings.
The elimination of lead from the engine bearings has the advantage of providing a more environmentally compliant engine bearing, and the substitution thereof with bismuth in the manner called for by the invention has the advantage of providing the same or better strength and wear/seizure resistant properties without requiring radical changes in the way engine bearings are made. As such, engine bearings prepared according to the invention are readily adaptable to new applications or existing applications that would otherwise call for copper-tin-lead bearings, and the manufacturer of bearings according to the invention can adapt to the making of such bearings without requiring new or different manufacturing equipment, and perhaps eliminating some of the steps and equipment normally associated with the manufacture of traditional copper-tin-lead bearings.
According to a further aspect of the invention, particular benefits have been realized when the copper-tin-lead bismuth powder metal material is produced from a blend of gas-atomized copper-tin-bismuth powder and water-atomized copper-tin-bismuth powder. Again, while not entirely understood, it is believed that the process by which the powders are made contributes to the mobilization of the tin or the bearing surface.